Historically, the preferred materials to implement opto-electronic or photonic devices have been materials with direct band-gaps. This fact is related to the significantly higher efficiency of such materials for light emission and/or absorption, for opto-electronic processes involving transitions across the band-gap.
Silicon-based opto-electronic devices have consisted mainly of photo-detectors, such as Charge Coupled Devices (CCDs) and CMOS photo-gates or photo-diodes, for cost-sensitive products, and/or products requiring very large density of components.
Since the band-structure of silicon makes these devices suitable for operation across the visible spectrum, CCDs and CMOS image-sensors have been incorporated into digital cameras and camcorders. Another very important silicon opto-electronic device for cost sensitive applications, has been the Solar-Cell.
Photo-detectors for the technologically important wavelengths in the infrared, such as λ=1310 nm and λ=1550 nm, are not possible with silicon and have been fabricated with compound semiconductor materials, such as the III/V materials. These materials have also provided the solutions for high-performance opto-electronic applications, such as fiber optic communications, with heterojunction detectors and lasers.
In spite of continued research efforts, efficient light emission has remained an elusive prospect for silicon devices and technologies, due to the indirect band-gap of silicon. It was only fairly recently that efficient light absorption and emission, including lasers, were achieved with devices whose operating principles made them independent of the band-gap type and magnitude. Examples of these devices are the Quantum Well Infrared Photo-detector (QWIP) and the Quantum Cascade Laser (QCL) respectively.
Band-gap engineering is also possible in the silicon materials system with Si1-xGex (SiGe) and/or Si1-x-yGexCy (SiGeC) films strained to the silicon lattice. However, the band offsets are small compared to those of the III/V materials, thereby limiting the energy of the opto-electronic transitions between bound states, that is, limiting the range of wavelengths for device operation.